Capacitive liquid sensor

ABSTRACT

A capacitive level gauge for placement in a container (12) determines the level of substance (14) in the container (12). The gauge (10) includes a measurement capacitor (C1) for measuring level and a reference capacitor (C2) for determining dielectric constant of the substance (14). A controller (34) is responsive to the capacitors (C1, C2) for producing a level signal which simultaneously indicated the level and dielectric constant of the material. The level signal includes a frequency which is representative of dielectric constant and a pulse width representative of level. The gauge (10) supports a first pair of parallel conductive members (26, 28) to establish the measurement capacitor (C1) and a second pair of parallel conductive members (28, 32) spaced along the gauge and below the measurement capacitor (C1) to establish the reference capacitor (C2).

TECHNICAL FIELD

The invention relates to a capacitance gauge which measures level bysensing change in capacitance between two plates.

BACKGROUND OF THE INVENTION

Capacitance gauges have been used in environments such as air craft, formeasuring the level of fuel in a wing tank. Capacitance gauges have nomoving parts and are therefore very reliable, especially in hostileenvironments were vibration and temperature extremes render mechanicallevel sensors useless. Capacitance gauges allow for relatively simplecompensation of various tank shapes, where linear changes in fluid leveldo not correspond to linear changes in fluid volume.

Capacitance level sensors comprise two plates which establish acapacitor. All capacitance level sensors are based on the fact thatelectrical capacitance between two electrodes or plates is described by:

    C(pF)=8.85×l0.sup.-2 S e (N-1)/d where:

S =area of one plate in cm²

N =number of plate

d =distance between plates (cm)

e =dielectric constant

Changes in dielectric constant in the medium separating the plates ofthe measuring capacitor will cause a change in measured capacity. Airhas a nominal dielectric constant equal to 1.0, and common oils orfluids such as kerosine or gasoline having nominal dielectric constantof 2.0. Such fluids rising between two parallel plates will increase thenet capacitance of the measuring cell as a function of fluid height. Thefluid being measured may not vary in dielectric constant, or changes inmeasured capacitance may erroneously be attributed to level changes.

Conventional capacitance level gauges can not handle fluids of varyingdielectric constant. Other fluids such as alcohol and water, which maybe present in varying amounts in modern automotive fuels are notcompatible with current capacitance gauges and act as a "poison" to thegasoline by artificially increasing the bulk dielectric constant of themixture by disproportionate amount. Ethanol and methanol have dielectricconstants of 24 and 31, respectively, and water has a dielectricconstant of 78. Relatively small concentrations of these additives togasoline will change the dielectric constant of the resulting mixture bya great amount. In most cases, the addition of 10% to 15% ethanol togasoline will raise the effective dielectric constant to approximately5.3.

U.S. Pat. No. 4,470,300, issued Sept. 11, 1984 in the name of Kobayashidiscloses a level gauge system for determining the alcohol concentrationin gasohol. The patent discloses a level gauge which uses a sensingcapacitor connected as an RC time constant to an astable miltivibratorfor determining the level. The frequency of the output from the astablemultivibrator indicates the capacitances of the sensor. The astablemultivibrator is mounted on the top of the fuel tank and is connected tothe capacitor so as the generate a signal which oscillates at afrequency depending on the capacitance of the capacitor. A perioddetector is connected to the oscillator in order to detect the period ofthe oscillatory signal therefrom. The oscillator includes the RC timeconstant arranged so that the oscillator generates pulses andfrequencies having a minimum value which is higher than thepredetermined reference value depending on the alcohol concentration ingasohol and the kind of the alcohol in gasohol.

The addition of alcohol to gasoline is the primary mechanism responsiblefor changes in the combustion characteristics, therefore, the alcoholconstant would be quite valuable. By continually monitoring the natureof fuel mixtures in the tank, the engine control computer could programthe engine operation for optimal performance in the minimum emission forany given fuel mix. A convenient way of determining the ratio of alcoholin gasoline is by monitoring the effective dielectric constant of thegasohol mixture. Incorporating a reference or normalizing capacitancecell into the level sensing capacitance gauge achieves compensation forchanges in dielectric constants due to temperature variation andmixture.

U.S. Pat. No. 4,590,575,issued May 20, 1986 in the name of Emplitdiscloses a time base system for determining the level of fluid whichutilizes a reference capacitance sensor and level capacitance sensor.The system is an on-line system wherein the time intervals frommultivibrators determines the capacitance. The system has a measurementprobe whose capacitance is a function of the level of substance in thetank and a reference probe whose capacitance is a function of thedielectric constant of the substance. Each probe is coupled to amultivibrator whose output frequencies are a function of the capacitancebetween the respective probe and the tank. The output signals of the twomultivibrators circuits are coupled to logic and switching circuits. Thecounter counts the number of input pulses and when a predeterminednumber have been counted, the counter inverts the binary state of theoutput and changes which sensor probe is transmitting the signal. Theoutput from the transmitter includes pulses whose time duration variesas a function of probe capacitance. The low level pulse segmentscorrespond to the frequency of the reference multivibrator and the highlevel pulses represent the output of the frequency of the levelmultivibrator. When the output signal is received, the signal isseparated into two time intervals signals, one representing each probe.The duration of the segments used by the counter produced for themicroprocessor and input representatives of the capacitance of themeasurements and thereby the level of substance in the tank. The problemwith such a system is that the sizes of the tank must be known sincecapacitance is established between a probe and a tank. Furthermore, thesignal produced alternates between the reference signal and themeasurement signal with time delays therebetween resulting in a lessaccurate system and slower responding system

None of the prior art accurately produces a signal which simultaneouslycomprises information of level and dielectric constant, or compensatedlevel measurement for any shape of container.

STATEMENT OF THE INVENTION AND ADVANTAGES

The invention is a method and capacitance level gauge assembly forplacement in a container to determine the amount or level of a substancetherein and the method of gaging the amount or level of substance in thecontainer. The assembly includes a sensing means having a first andsecond end for measuring the level of the substance between the firstand second ends. The sensor means includes measurement capacitance meansextending between the first and second ends comprising at least twoparallel conductive members to establish an electrical capacitancerepresentative of level, and reference capacitance means connected atthe second end comprising at least two parallel conductive members toestablish an electrical capacitance representative of dielectricconstant. Measuring means is responsive to the level capacitance andreference capacitance for producing a level signal simultaneouslyrepresenting both level and dielectric constant.

An advantage of this invention is that its use does not require apredetermined shaped container. Additionally, the level signalsimultaneously indicates the level capacitance and reference capacitancefor accurate indication of the level.

DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated asthe same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a perspective view of a first embodiment of the sensing means;

FIG. 2 is a perspective view of a second embodiment of the sensingmeans;

FIG. 3 is a perspective view of a third embodiment of the sensing means;and

FIG. 4 is a schematic diagram of the subject invention.

DESCRIPTION OF PREFERRED EMBODIMENT

A capacitance level gauge assembly for determining the amount or levelof substance in a container is generally shown at 10 in FIG. 1. Theassembly 10 determines the level of the substance 14 in the container 12by compensating for change in dielectric constant. The substance 14 maybe comprised of any materials, such as a solid or liquid, as long as ainterface 16 is established above and below which there is a change indielectric constant. In the preferred embodiment, the interface 16 isestablished between air and fuel of a vehicle in a fuel tank 12. Airgenerally has a dielectric constant of 1.0, whereas fuels and alcoholshave a greater dielectric constant.

The assembly 10 comprises a sensing means 18 having a first 20 andsecond 22 end for measuring the level of substance 14. The sensing means18 includes measurement capacitance means 24 extending between the first20 and second 22 ends comprising at least two parallel conductivemembers 26, 28 establishing a level capacitor Cl having an electricalcapacitance representative of the level in the container 12. The sensingmeans 18 also includes reference capacitance means 30 connected at thesecond end 22 comprising at least two parallel conductive members 26, 32establishing a reference capacitor C2 having an electrical capacitancerepresentative of dielectric constant. Measuring means 34 is responsiveto the level capacitor Cl and the reference capacitor C2 for producing alevel signal simultaneously representing both the level and dielectricconstant of the substance 14. The measuring means 34 determines thelevel as a function of dielectric constant of the substance 14 and levelsubmerged in the substance 14. An interface 16 is located between thefirst 20 and second 22 ends of the measurement capacitance means 24,which interface 16 moves longitudinally along the measurementcapacitance means 24 changing the capacitance and therefore the levelsignal.

FIGS. 1 through 3 illustrate three embodiments of the sensing means 18.Generally, the reference capacitance means 30 of the sensing means 18 isdisposed at the second end 22 which is the base or lower end of thesensing means 18 so that it will always be submerged in the fluid orsubstance 14. The measurement capacitance means 24 is arranged so thatit senses the height or level of fluid 14 within the tank or container12. Therefore, the measurement capacitance means 24 extends a distanceto allow the container 12 to become full and empty while the interface16 moves along the measurement capacitance means 24 between the firstend and reference capacitance means 30.

As illustrated in FIG. 1, the first embodiment of the sensing means 18comprises multiple parallel wires 26, 28, 32 which form alternating"plates" of two separate sensing capacitors Cl, C2 extending the lengthof the sensing means 18. The reference capacitance means 30 is submergedbelow the substance 14 at the lower end or second end 22 of the sensingmeans 18 and comprises multiple parallel wires 28, 32 between a baseinsulator 36 and a intermediate insulator 38. The measurementcapacitance means 24 extends a distance which will incorporate allchanges in the level of the substance 14 and comprises multiple parallelwires 26, 28 between the intermediate insulator 38 and a ceilinginsulator 40. The insulators 36, 38, 40 are generally circular rings ordisks for receiving a support post 44 therethrough. The support post 44maintains the insulator rings 36, 38, 40 at their respective locations.The multiple parallel wires 26, 28, 32 are spaced apartcircumferentially about the support post 44 and into the insulators 36,38 40. The parallel wires 26, 28, 32 are insulated. Alternate wires ofthe measurement capacitance means 24 and reference capacitance means 30are connected to ground and the remaining to the control means 34. Inthe preferred embodiment, there are twenty-four (24) parallel wires onlyeight are shown. Twelve (12) wires 28 are grounded, and four wires 26are connected comprising a plate of the measurement capacitor Cl andfour wires 32 are connected together comprising a plate of the referencecapacitor C2, which wires 26, 28, 32 are connected to the control means34.

FIG. 2 illustrates the second embodiment of the sensing means l8' whichinclude two continuous adjacent wires wound in a two or dual leadhelical configuration to establish the "plates" of both capacitors Cl,C2. The reference capacitance means 30 or cell is placed at the base orsecond end 22' of the sensing means 18 and shares the interwound groundor common electrode 28' with the measurement capacitance means 24'. Theadvantage of the helical design is the ability to compensate for nonlinear volume-level relationships in some tanks 12 by varying the pitchof the wire windings 26', 28', 32'. The sensing means 18' comprises agenerally cruciform or cross-shaped support 46 of insulating materialwith a control, housing 47 attached at the upper or first end 20'. Thecommon or ground wire 28' is wound to extend the length of the support46 and the second wire 32' is wound at the base of second end 22' toproduce the reference capacitor C2 and a third wire 26' is wound betweenthe base and the upper end to produce the measurement capacitor C1 Thewires are insulated.

The third embodiment of the sensing means 18 is illustrated in FIG. 3.The sensing means 18 includes two outside parallel, flat ground plates28" and a measurement plate 26" and reference plate 32" locatedtherebetween. The reference plate 32" is located at the second end 22'or base of the sensing means 18, and the measurement plate 26" islocated thereabove and spaced from the reference plate 32". The controlmeans 34 may be housed 47' at the upper end or first end 20'of thesensing means 18. The plates may be secured to a support housing 45extending the length of the sensing means 18 to support the outsideedges of the plates 26", 28", 32". The housing 45 is insulated and maycontain wires extending between the plates 26", 28", 32" and the controlmeans 34.

By utilizing the designs of one of the three embodiments, common designfeatures have been developed. The support structure must allow fluid todrain freely. Liquid captured under the wires or plates will obviouslygive erroneous reading. Wire or plate supports must be designed tominimize the parasitic capacity of the sensing capacitors C1, C2 andtheir leads, so that the gauge 10 is more sensitive to change in thedielectric constant of the medium surrounding the wire electrodes.Therefore, as much of the sensing wires or plates 26, 28, 32 needs to befreely suspended in air or fluid. Sensor support structures which areimmersed in gasohol, can not be fabricated out of materials which absorbwater such as nylon. Even small amounts of water absorbed by the plasticfrom the fuel, grossly affected the capacitance readings since thedielectric constant of water is nearly forty (40) times that ofgasoline. Non hygroscopic plastics such as high density polyethelyne andpolypropelene aremore suitable.

Wires or plates used as capacitor electrodes 26, 28, 32 must beinsulated to prevent electrical conduction through imperfectdielectrics, such as water bearing gasohols. Polyester-imide andepoxi-phenolic resin coatings offer the requisite long term solventresistance to various gasohol blends. In order to insure completesubmergence of the reference capacitance means 30 in minimal amounts offluid, the reference capacitance means 30 must be physically small, orat least short, and yet obtains efficient capacitance to providereliable dielectric constant readings of the fuel in the tank 12.lengths from the capacitance means 24, 30 to control means 34 must bekept short and direct, so that parasitic capacities is minimized. Theparallel plates 26, 28, 32 in the third embodiment may be made ofvirtually any conductive material, such as aluminum, steel, brass,copper, etc., and may be coated with similar insulating media as thewires.

The measuring means 34 is responsive to the measurement capacitancemeans 24 and reference capacitance means 30 to determine the level anddielectric constant, respectively. The measuring means 34 includes twotime base generators 48, 50. The period established by each capacitorCl, C2 in an RC or LC configuration is measured, and the resultantvalues are then multiplied with appropriate constants to yield acorrected fluid level. It is to be understood that several methods maybe used in measuring means 34, and the invention is not limited thereto.

In the preferred embodiment as shown in FIG. 4, the measuring means 34includes reference timing means 48 connected to the referencecapacitance means 30 for producing a reference signal having a timingcomponent t2 indicative of the capacitance. The reference timing means48 may comprise an astable multivibrator for producing the referencesignal having frequency representative of the capacitance of referencecapacitor C2. The reference timing means 48 includes reference resistivemeans 52 acting with the reference capacitor C2 for establishing an RCtime constant t2 of the reference signal.

The measuring means 34 includes level timing means 50 connected to themeasurement capacitance means 24 for producing a level signal having atiming component t1 indicative of the capacitance. The level timingmeans 50 receives the reference signal wherein the level signal includestwo timing components tl, t2 indicative of the capacitance of levelcapacitor Cl and of the reference capacitor C2, respectively. The leveltiming means 50 includes monostable multivibrator for producing thelevel signal having a pulse width tl indicative of the level capacitanceand the frequency indicative of the reference capacitance. The leveltiming means 50 includes level resistive means 54 for acting with alevel capacitor Cl for establishing an RC time constant t2 of the levelsignal.

The reference timing means 48, which is the astable multivibrator orfree running oscillator, has a frequency t2 which is determined by thevalue of the reference capacitance means 30 or reference capacitor C2.The output of the astable multivibrator triggers the level timing means50, or monostable multivibrator, wherein the output pulse width tl isestablished by the level sensing capacitor Cl or measurement capacitancemeans 24. The combination of the measurement capacitance means 24 andreference capacitance means 30 with the reference resistive means 52 andlevel resistive means 54, respectively, comprises the RC time constantfor producing frequency t2 and pulse width tl. The ratio of thereference capacitor C2 to the level capacitor Cl is adjustable and maybe a function of the geometry for a given tank. Experiments haveexhibited C2 to Cl capacity ratios of 1:5 to 1:10.

The fluid level values are a function of the output duty cycle (tl/t2).The output voltage is a time averaged voltage, proportional to the ratioof tl to t2 multiplied by the peak to peak voltage of the input pulses.The terms relating to the dielectric constant fall out of the equation,making the system self compensating when the appropriate proportionalityconstants are applied.

The measuring means 34 further includes voltage follower and low passfilter means 62 connected to a meter means 60. The meter means 60receives the level signal and produces a visual output of actual levelor volume of the substance 14 in the container which is compensated forchanges in dielectric constant. The meter means 60 may include amultiplier factor to determine the volume of substance.

Alternatively, the measuring means 34 may include rectifier means forreceiving and rectifying a level signal producing a rectified signal.Filter means receives and filters the rectified signal producing afiltered signal. A meter means receives the filtered signal and visuallydisplays the magnitude of the filter signal. The meter means may be ananalog meter which is directly responsive to the magnitude of thefiltered signal. The measuring means 34 may alternately include digitalmonitor means for receiving the level signal and for determiningdielectric constant by the frequency of the level signal and determiningthe level by the level signal.

The measuring means 34 may also include a monitor with the discretelogic or directly with a microprocessor which receives the output of thelevel timing means 50. The repetition rate or frequency output pulseswould yield the dielectric constant correction parameter, while the dutycycle measurement plus the influence of the first parameter would be afunction of fluid height. With direct digital processing, the two timingfunctions can be totally independent.

The invention also includes method of gauging the amount or level ofsubstance 14 in the container. The method includes the steps of sensinga first capacitance indicative and a sensing a second capacitanceindicative of dielectric constant. A level signal is produced having afrequency indicative of one of the first capacitance and secondcapacitance and a pulse width indicative of the other of the firstcapacitance and second capacitance. More specifically, the referencesignal has frequency indicative of dielectric constant. The level signalis produced which includes producing a level signal having pulse widthindicative of the level and frequency indicative of dielectric constant.

The invention has been described in an illustrative manner, and it is tobe understood that the terminology which has been used is intended to bein the nature of words of description rather than of limitation.Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims whereinreference numerals are merely for convenience and are not to be in anyway limiting, the invention may be practiced otherwise than asspecifically described.

What is claimed is:
 1. A capacitive sensor assembly for placement in a container to determine the level of substance therein, said assembly comprising; sensing means (18) having a first and second end (22) for measuring the level of substance between said first and second end; said sensing means including measurement capacitance means (24) extending between said first and second ends comprising at least two parallel conductive members to establish an electrical capacitance representative of level, and reference capacitance means (30) connected at said second end (22) comprising at least two parallel conductive members to establish an electrical capacitance representative of dielectric constant; measuring means (34) responsive to said level capacitance for producing a reference signal representing dielectric constant and reference capacitance for producing a level signal representing level of the substance said measuring means (34) including reference timing means (48) connected to said reference capacitance means (30) for producing a reference signal having a timing component indicative of said reference capacitance, and level timing means (50) independent from said reference timing means (30) and connected to said measurement capacitance means (24) for producing a level signal having a timing component indicative of said level capacitance.
 2. An assembly as set forth in claim 1 further characterized by said reference timing means (48) comprising a free running oscillator having the timing component indicative of said reference capacitance and independent of said level capacitance, and said level timing means (50) comprising a second free running oscillator having the timing component indicative of said level capacitance.
 3. A capacitive level gauge assembly for placement in a container to determine the amount or level of a substance therein, said assembly comprising; sensing means (18) having a first and second end (22) for measuring the level of substance between said first and second end; said sensing means including measurement capacitance means (24) extending between said first and second ends comprising at least two parallel conductive members to establish an electrical capacitance representative of level, and reference capacitance means (30) connected at said second end (22) comprising at least two parallel conductive members to establish an electrical capacitance representative of dielectric constant; measuring means (34) responsive to said level capacitance and reference capacitance for producing a level signal representing both level and dielectric constant; said measuring means (34) including reference timing means (48) connected to said reference capacitance means (30) for producing a reference signal having a timing component indicative of said reference capacitance and independent of said level capacitance, and level timing means (50) connected to said measurement capacitance means (24) for receiving said reference signal and producing a level signal having a timing component indicative of said level capacitance and the timing component indicative of said reference level capacitance.
 4. An assembly as set forth in claim 1 further characterized by said reference timing means (40) comprising astable multivibrator for producing said reference signal having frequency representative of said reference capacitance.
 5. An assembly as set forth in claim 4 further characterized by said level timing means (50) comprising monostable multivibrator for producing said level signal having pulsewidth indicative of said level capacitance and said frequency indicative of said reference capacitance.
 6. An assembly as set forth in claim 5 further characterized by said reference timing means (48) including reference resistive means (52) for acting with said reference capacitance for establishing a time constant of said reference signal.
 7. An assembly as set forth in claim 6 further characterized by said level timing means (50) including level resistive means (54) for acting with said level capacitance for establishing a time constant of said level signal.
 8. A capacitive level gauge assembly for placement in a container to determine the amount or level of a substance therein, said assembly comprising; sensing means (18) having a first and second end (22) for measuring the level of substance between said first and second end; said sensor means including measurement capacitance means (24) extending between said first and second ends comprising at least two parallel conductive members to establish an electrical capacitance representative of level, and reference capacitance means (30) connected at said second end (22) comprising at least two parallel conductive members to establish an electrical capacitance representative of dielectric constant; said assembly characterized by control means (34) responsive to said level capacitance and said reference capacitance for producing a level signal having a frequency indicative of one of said level capacitance and said reference capacitance and a pulse width indicative of the other of said level capacitance and said reference capacitance.
 9. A capacitive sensor assembly for placement in a container to determine the level of substance therein, said assembly comprising; measurement capacitance means (24) comprising at least two parallel conductive members establishing a level capacitance which varies as a function of level, reference capacitance means (30) comprising at least two parallel conductive members establishing a reference capacitance which varies as a function of dielectric constant of the material, and longitudinal support means having a first end (20) and a second end (22), said second end (22) for supporting said reference capacitance means (30) longitudinally extending along said support means immersed in the substance (14) and said measurement capacitance means (24) longitudinally spaced apart from said reference capacitance means (30) and longitudinally extending along said support means from said second end (22).
 10. An assembly as set forth in claim 9 further characterized by said support means including a base insulator located at said second end and an intermediate insulator located between said first and second ends and a ceiling insulator located at said first end, said measurement capacitance means (24) comprising a plurality of parallel wires connected between said intermediate insulator and said ceiling insulator and said reference capacitance means (30) comprising a plurality of parallel wires connected between said intermediate insulator and said base insulator.
 11. An assembly as set forth in claim 9 further characterized by said support means being cruciform shaped and said measurement capacitance means (24) comprising two parallel wires wound about said support means in helical configuration from said first end to said second end and said reference capacitance means (30 comprising two parallel wires wound about said support means in helical configuration at said second end.
 12. An assembly as set forth in claim 9 further characterized by said measurement capacitance means (24) comprising two parallel plates extending from said first end toward said second end and said reference capacitance means (30) comprising two parallel plates at said second end, said support means supporting said plates along edges of said plates. 